1. Field of the Invention
The present invention is directed to systems, methods, and/or apparatus that utilize the output of an audio bridge-tied-load (BTL) amplifier as the source of both power and audio signals to perform audio amplification not possible with the BTL amplifier alone. Accordingly, the general objects of the invention are to provide novel systems, methods, and/or apparatus of such character.
2. Description of the Related Art
The majority of automobiles produced today have audio systems specifically tailored to the vehicle by the vehicle manufacturer. These audio systems typically have the capability to receive audio information from a variety of audio media sources such as radio, CD, MP3, etc. The audio media is converted to electrical audio signals which are amplified by one or more amplifiers, and then applied to speakers that produce audible sounds.
Many vehicle manufacturers utilize BTL (bridge-tied-load) amplifier configurations. The term bridge is used to describe the output stage of the BTL amplifier, in which the output is arranged in a full bridged configuration. The full bridge configuration can be viewed as two separate amplifier stages sharing the same voltage source and audio input signal. One of the amplifiers audio input stage is configured as non-inverting and the other's audio input stage is configured as inverting. The non-inverting amplifier forms the non-inverting output of the BTL amplifier. The inverting amplifier forms the inverting output of the BTL amplifier. The speaker load is connected between the two outputs forming a bridge. Both outputs produce opposite output voltages simultaneously which results in a balanced BTL amplifier output. The positive speaker input is connected to the non-inverting output of the amplifier. The negative speaker input is connected to the inverting output of the amplifier. The amplifier output signal presented to the speaker is a differential signal with equal and opposite polarity signals delivered to each side of the load.
When no output signal is present, the non-inverting and inverting outputs of the BTL are at a DC voltage between the vehicles battery voltage and ground. Usually the DC value is half of the vehicles primary DC voltage. Typically the primary DC voltage is provided by the vehicle's battery. With no output signal present, a typical 13.8 Vdc vehicle battery would yield around 6.9 Vdc at both the non-inverting and inverting BTL amplifier outputs with respect to vehicle battery ground. With no output signal present, no current will flow through the speaker load due to the equal DC voltage found on both output terminals of the BTL amplifier. When audio signal is present, the power output devices of the BTL amplifier conduct through the speaker load alternately between positive and negative transitions of the audio signal.
The regulated nominal DC output voltage of a vehicle's electrical system when the vehicle engine is running is around 13.8 Vdc. This DC value can either increase or decrease depending on the electrical load and or RPM (revolutions per minute) of the vehicle's engine. Typically the DC output voltage of the vehicle is regulated to a value below 14.4 Vdc. When the vehicle's engine is not running the nominal DC voltage is typically an unregulated 12.5 Vdc.
In stock audio systems, the BTL amplifier's source voltage is generally the same value as the vehicle battery voltage. Utilizing the full bridge output method yields quadruple the amplifier output power compared to half bridge amplifier configurations wired into the same value load. The output power level produced by the stock BTL amplifier, powered by the primary vehicle DC voltage, into the stock speakers is generally considered adequate by the vehicle manufacturer. Every conventional BTL amplifier for use with a vehicle has an output current capacity and a dynamic output power capacity that are, ultimately, limited by the vehicle's battery.
FIG. 1A represents a typical configuration for a conventional Bridged-Tied Load (BTL) amplifier 10. As shown, two essentially separate amplifiers are joined together through a load 16 at terminals 12 and 14. The audio signal is applied to the non-inverting input of one amplifier AMP 1 and the inverting input of the other amplifier AMP 2. As is known, each of the balanced audio signals of the BTL amplifier has a zero-crossing point at which the polarity of the balanced audio signals reverses.
Both amplifiers AMP1 and AMP 2 share a common DC voltage source. In a typical conventional application such as an automobile audio amplifier, the DC voltage source is a battery voltage. It is also shown that the reference point for both amplifiers is half of the source voltage (HALF BATTERY VOLTAGE). The half source voltage is applied to both sides of the load when no signal is present. When signal is applied, both bipolar positive and negative audio signals 18 and 20 alternate between the source voltage and ground about a DC midpoint (7.2 volts) as shown in FIGS. 1B and 1C. When driving a resistive load, the output current level would be proportional to the output voltage level.
FIGS. 1D and 1E show total worst case power dissipation in the output devices of the BTL amplifier when connected through a 4 Ohm resistor driven for 3 cycles at 1 kHz. FIG. 1D (waveform 22) shows the 1 kHz output voltage waveform of the BTL amplifier. FIG. 1E (waveform 24) shows the total worst case dissipation in the output transistors for the output waveform shown in 1D. The total average power dissipation of M1, M2, M3, and M4 in FIG. 1A under this condition is 10 Watts. When the BTL amplifier is connected to the inputs of the invention, M1 and M3 source power and dissipate heat. M4 and M2 do not source power and their heat contribution to the BTL amplifier heat sink is negligible. The total average power dissipation of M1 and M2 with the invention connected to the output of the BTL amplifier, and the same 3 cycle worst case conditions, is 13 Watts.